...Although the "R/T" discussion is meaningless...the explanation for that is simply that Kirk's middle name began with an "R" when WNMHGB was made, but somewhere along the way the creators/producers/writers of Star Trek changed it to a "T". It's not the first or last inconsistency within TOS.
Ummm...
I think we all know what the real world reasons could have been.
Any time this is brought up, it's fans speculating on a "why" having to do with the in-story continuity. That's not meaningless. It's a mental exercise involving something we enjoy.
It's sort of like discussing the motivations of characters in a classic novel. Sorry, but characters in novels don't have motivations. They do what the writer makes them do. Still, that doesn't mean we can't discuss why a character MIGHT have done what they did. It's a bit of fun.
Much the same with Star Trek and the WNMHGB tombstone. WHY did Gary put in the middle initial R, in light of what we later hear Kirk's middle initial to be? Personally, I think he might have just been getting in another dig in what he thought were Kirk's last moments. Maybe a leftover from an old joke or an old nickname Kirk was known by.
Don't stifle "playing with Trek". People enjoy it and have the right to!